Electroless plating, particularly autocatalytic electroless plating, generally uses a plating solution containing a reducing substance for supplying electrons and a metal ion source for plating. The plating solution is adjusted to a predetermined pH using ammonia as a pH adjuster. The reducing substance used in the plating solution varies depending on the metal used for plating. Copper plating uses formaldehyde. Nickel plating uses phosphites, hypophosphites, tetrahydroborates, or hydrazines. Gold plating uses tetrahydroborates, ascorbic acid, thiourea, or other reducing substances.
Thus, electroless plating processes produce plating wastewater containing such reducing substances, ammonia, and metal ions. An ammonia molecule has a lone electron pair on its nitrogen atom, so that ammonia acts as a ligand to metal ions in plating wastewater. Accordingly, at least a part of ammonia and metal ions are present in the form of a complex thereof in plating wastewater.
There is proposed a method for treating plating wastewater containing ammonia and metal ions by adding an alkali metal salt of phosphoric acid to the plating wastewater, adjusting it to a pH of 6.5 to 8.5 to precipitate a poorly soluble metal phosphate, and removing the precipitate (see Patent Literature 1).
Also proposed is a method for treating plating wastewater containing phosphorous acid as a reducing substance by oxidizing the phosphorous acid into orthophosphoric acid with hydrogen peroxide and then adding a calcium salt to precipitate calcium phosphate (see Patent Literature 2).
Patent Literature 1: Japanese Patent H3-44839 B
Patent Literature 2: Japanese Patent Publication 2004-284908 A
The method of Patent Literature 1 has a problem in that the treatment cost rises considerably due to the use of a treatment chemical containing an alkali metal salt of phosphoric acid, which is relatively expensive. The method further has a problem in that a large amount of sludge are produced. The method also has a problem in that large equipment including a phosphorus removal unit installed downstream is required since phosphorus remains in the treated water.
The method in Patent Literature 2 has a problem in that, during sedimentation of precipitates such as calcium phosphate after a step of oxidizing the reducing substance with hydrogen peroxide, the precipitates float due to oxygen gas produced by the self-decomposition of the hydrogen peroxide remaining after the oxidation step. These precipitates do not settle rapidly and thus decrease the treated water quality.